There are known, as a method for producing a flexible substrate in which a metal layer and a polyimide layer are laminated, for example, a method of bonding a metal foil and a polyimide film with an adhesive interposed therebetween, a method of heating and pressing a metal foil and a polyimide film, a method of depositing a metal on a polyimide film, and a method of applying a polyimide solution or a precursor of polyimide on a metal foil.
Furthermore, it has been suggested that when a thin film transistor (hereinafter, may be referred to as TFT) is produced on a plastic film, a film formed of an inorganic material is provided on the plastic film in order to prevent detachment of the TFT from the plastic film (see, for example, Patent Document 1 and Patent Document 2).
In regard to the method for producing a flexible substrate in which a metal layer and a polyimide layer are laminated, in the case of the method of using a metal foil, for example, when the metal film is a rolled foil, surface irregularities due to rolling streaks exist on the surface. Even in the case where the metal foil is an electrolytic foil, surface irregularities exist on the surface. Therefore, there is a problem that when a TFT is produced on a flexible substrate, the electrical performance of the TFT is deteriorated. Also, in the case of the method of using a polyimide film, since polyimide films usually contain particles for the purpose of enhancing anti-blocking properties, surface irregularities exist on the surface. Thus, similarly, there is a problem that when a TFT is produced on a flexible substrate, the electrical performance of the TFT is deteriorated.
Thus, a polyimide layer having satisfactory surface smoothness is desired.
However, even if a polyimide layer having satisfactory surface smoothness has been obtained, since polyimide has relatively high hygroscopic properties, there is a problem that in a process of producing a TFT on a flexible substrate, the TFT undergoes cracking or detachment as the dimension of the polyimide layer changes due to moisture. Furthermore, the dimension of the polyimide layer changes not only because of moisture but also because of heat, similarly, there occurs a problem of cracking or detachment of the TFT.
Meanwhile, polymeric materials are used in various manufactured goods for daily life due to their characteristics such as easy processability and light weight. Polyimides that have been developed by DuPont Company, the U.S.A. in 1955 have excellent heat resistance, and therefore, development has been in progress, such as an investigation being conducted on the application of polyimides to the field of aerospace. Since then, detailed investigations have been conducted by many researchers, and it has become clear that polyimides exhibit top-class performance even in organic materials in terms of heat resistance, dimensional stability and insulating characteristics. Thus, polyimides have been applied not only to the field of aerospace but also to insulating materials for electronic components. At present, polyimides are actively used in the chip-coating films in semiconductor devices, the base material for flexible printed wiring boards, and the like.
Polyimides are polymers that are synthesized primarily from diamines and acid dianhydrides. When a diamine and an acid dianhydride are allowed to react in a solution, a polyamide acid (polyamic acid), which is a precursor of polyimide, is formed, and thereafter, the polyamide acid is converted to polyimide through a dehydration ring-closure reaction. Generally, since polyimides lack solubility in solvents and are difficult to process, in many cases, a desired shape is produced while the compound is in the form of a precursor, and then the precursor is converted to polyimide by being subjected to heating. Many of polyimide precursors are unstable to heat or water, and the precursors tend to have poor storage stability, such as requiring refrigerated storage. In view of this point, there have been developed polyimides in which a skeleton having excellent solubility is introduced into the molecular structure, or the molecular structure is subjected to modification such as reduction of the molecular weight, so that molding or coating can be achieved by first obtaining a polyimide and then dissolving the polymer in a solvent. However, when these polyimides are used, the polymers tend to deteriorate in the film properties such as heat resistance, chemical resistance, the coefficient of linear thermal expansion, and the coefficient of hygroscopic expansion, as compared with systems using polyimide precursors. Therefore, systems using polyimide precursors and systems using solvent-soluble polyimides can be distinguished for use according to the purpose.
A polyimide film is formed, in the case of using a solvent-soluble polyimide, by applying a polyimide resin composition on a substrate, and heat treating the composition to evaporate the solvent; and in the case of using a polyimide precursor, a polyimide film is formed by applying a polyimide resin composition on a substrate, heat treating the composition to evaporate the solvent, and then further heating the polymer to subject the polymer to imidization and thermal cyclization. In this case, when the polyimide resin composition is applied, if the wettability to the base material is poor, it is difficult to uniformly apply the resin composition on the base material, so that there occurs a problem in the flatness of the surface after film formation. Also, if the effect is conspicuous, there is a problem that repelling or foaming may occur, pinholes may be formed in the film, or the like.
In order to address the problems described above, it has been proposed to add a surfactant formed from a silicone oil into the polyimide resin composition (see, for example, Patent Document 3). According to this technology, it is believed that by adding a surfactant, air bubbles are not easily generated when a film is formed, and a decrease in the film uniformity due to air bubbles, or the generation of pinholes can be suppressed.
In recent years, polyimides are widely used as insulating materials for electronic components, and various performances are now demanded. Among them, particularly in a substrate for a thin film element in which a metal base material and an insulating layer containing polyimide are laminated, which substrate is used in thin film elements such as thin film transistors (TFT), thin film solar cells, and electroluminescent elements (hereinafter, electroluminescence may be referred to as EL), the thin film element unit formed on the insulating layer is thin, and also, when the metal base material is a rolled foil, surface irregularities due to rolling streaks exist on the surface, while when the metal base material is an electrolytic foil, surface irregularities also exist on the surface. Therefore, there is a problem that the characteristics of the thin film element are deteriorated due to the surface irregularities. Thus, it is demanded to improve the surface smoothness of the substrates for thin film elements.
However, in the case of applying a polyimide resin composition on a metal base material and thereby forming an insulating layer, when the polyimide resin composition is applied in the manner such as described above, uniform application is difficult, and repelling or foaming occurs. Also, surface irregularities exist on the surface of the metal base material, there is a problem that the uniformity of the film is markedly deteriorated.
It is believed that in the technique of using a polyimide resin composition to which a surfactant has been added as described in Patent Document 3, a film having excellent surface smoothness regardless of the surface irregularities of the substrate surface, can be formed. However, in the method of adding additives to such a polyimide resin composition, there remain various problems such as the compatibility of the additives with the polyimide resin composition, and the deterioration of the characteristics such as heat resistance of the film caused by the additives.
Furthermore, there is a problem in polyimide films that craters or pinholes due to air bubbles are generated. When craters or pinholes are present, the performance is deteriorated, and it is difficult to use polyimide films as insulating layers of electronic components.
Various factors can be considered as the causes for the generation of air bubbles in a film, but one of the factors may be a polyimide resin composition used in the formation of polyimide films. When a polyimide resin composition is prepared, air bubbles are incorporated into the polyimide resin composition, and these air bubbles remain in the film.
In order to remove air bubbles that have been incorporated into the polyimide resin composition, there has been proposed a method of degassing a polyamic acid varnish or a polyimide varnish (see, for example, Patent Documents 4 to 6). As a method of degassing a polyamic acid varnish or a polyimide varnish, for example, Patent Document 4 discloses a reduced pressure degassing method, a thin film type reduced pressure degassing method, and a centrifugal thin film degassing method; Patent Documents 4 and 5 disclose degassing methods using ultrasonic waves; and Patent Documents 4 and 6 disclose degassing methods of filtering using a filter.
As explained in the above, in a substrate for a thin film element in which a metal base material and an insulating layer containing polyimide are laminated, which substrate is used in thin film elements such as TFTs, thin film solar cells, and EL elements, since the thin film element unit formed on the insulating layer is thin, there is a risk that the fine surface irregularities of the surface of the substrate for a thin film element may deteriorate the characteristics of the thin film element. Therefore, it is demanded to improve the surface smoothness of the substrate for a thin film element. For example, in the case of a TFT, when there are fine surface irregularities in the semiconductor layer of the TFT, particularly in the underlying layer of a channel-forming region, that is, when fine surface irregularities exist on the surface of the insulating layer containing polyimide, the mobility of the TFT is markedly decreased, or a leak current flows, and the characteristics of the TFT are critically affected. Furthermore, the yield is decreased due to the surface state of the insulating layer containing polyimide.
The generation of craters or pinholes that are caused by air bubbles, on the surface of the insulating layer containing polyimide, can be suppressed through degassing of the polyimide resin composition. However, in the related art, as described in Patent Documents 4 and 6, those air bubbles having a size in the order of micrometers are considered to be problematic, but no investigation has been carried out on the air bubbles having a size in the order of nanometers. On the other hand, since the thin film element unit is thin in a substrate for a thin film element, the air bubbles having a size in the order of nanometers that affect the thin film element unit cause a problem. For example, in a TFT, those air bubbles in the nanometer order that affect the channel-forming region, cause a problem.
Here, foam in a liquid is in a state where a gas is mixed in a liquid while being in a gaseous form. This foam is not only incorporated from the outside, but is also very frequently generated from the liquid. On the other hand, a dissolved gas means a gas that is dissolved in a liquid, and this dissolved gas is not visible to the eye unlike the case of the foam.
The amount of a gas dissolved in a liquid varies with the type of the liquid, temperature or pressure, and the material to be wetted, and the dissolved gas in excess of the saturation amount is converted to foam and emerges. That is, even a liquid in a state without any foam may generate foam when the temperature or pressure changes. On the other hand, even if foam is present in the liquid, when the liquid is at a predetermined temperature or pressure, or when the amount of gas dissolved therein is lower than the saturation value, the foam is dissolved in the liquid. That is, simply removing the foam is not sufficient, and it is important to remove any dissolved gas.
If a dissolved gas is present in a polyimide resin composition in an amount close to the saturation amount, during the processes of application or the like, when the temperature or the pressure changes, the gas in excess of the saturation amount appears in the form of foam. If the gas in excess of the saturation amount is in a small amount, since it is difficult for the air bubble size to grow, there is no serious problem in view of the reduction of micrometer-sized air bubbles.
However, in order to control the smoothness to the order of nanometers in the surface of the insulating layer, it is necessary to suppress the generation of those air bubbles having a size in the order of nanometers. Accordingly, it is necessary to reduce the dissolved gas content in order to avoid the presence of any dissolved gas in excess of the saturation amount. Therefore, in order to prevent the generation of air bubbles having a size in the order of nanometers which exert adverse influence on the surface smoothness of the insulating layer in a substrate for a thin film element, it is very important to maintain the dissolved gas content in the polyimide resin composition to a value lower than the saturation amount.